Chemical Principles: Understanding Matter and Its Transformations

The Nature of Matter and Its Changes

Chemistry is the scientific discipline dedicated to studying the nature of matter and the changes affecting its composition and properties.

Physical vs. Chemical Changes

• Physical changes do not alter the fundamental nature or chemical identity of substances. Examples include melting ice or boiling water.
• Chemical changes (or chemical reactions) result in the formation of new substances with different chemical properties. Examples include burning wood or rusting iron.

Classifying Matter

Pure Substances

A pure substance has a definite and constant composition, characteristic physical and chemical properties, and cannot be separated into other substances through physical methods.

Mixtures

A mixture has a variable composition and properties that are related to its components. Mixtures can be classified as:

• Homogeneous mixtures: Have a uniform composition throughout (e.g., saltwater).
• Heterogeneous mixtures: Have a non-uniform composition, with visibly distinct parts (e.g., sand and water).

Separation of Mixtures

The components of mixtures can be separated by various physical methods because these methods do not alter the chemical nature of the substances. Common separation techniques include:

• Filtration
• Centrifugation
• Crystallization
• Distillation
• Chromatography

Building Blocks of Matter

Elements

An element is a fundamental substance that cannot be decomposed into simpler substances by chemical means. Elements combine with other elements to form compounds.

Compounds

A compound is a substance formed by the chemical combination of two or more different elements in fixed proportions. Compounds can be decomposed into their constituent elements by chemical reactions.

Fundamental Laws of Chemistry

Law of Conservation of Mass (Antoine Lavoisier)

In any closed system, the total mass of the reactants before a chemical reaction must equal the total mass of the products after the reaction. Mass is conserved.

Law of Definite Proportions (Joseph Proust)

When two elements combine to form a particular compound, they always do so in the same fixed ratio of masses, regardless of the source or method of preparation.

Law of Multiple Proportions (John Dalton)

If two elements can combine to form more than one compound, then the masses of one element that combine with a fixed mass of the other element are in a ratio of small whole numbers.

Law of Combining Volumes (Joseph Louis Gay-Lussac)

When gases react together to form other gases, and all volumes are measured at the same temperature and pressure, the ratio between the volumes of the reactant gases and the products can be expressed in simple whole numbers.

Early Atomic and Molecular Theories

Dalton's Atomic Theory

Key postulates of Dalton's theory include:

• Matter is composed of extremely small, indivisible particles called atoms.
• Atoms of a given element are identical in mass and properties. Atoms of different elements differ in mass and properties.
• Atoms cannot be created or destroyed in a chemical reaction.
• Atoms combine in simple whole-number ratios to form compounds.

Avogadro's Molecular Theory

Avogadro proposed that gas particles are not necessarily individual atoms but can be groups of atoms called molecules, each with a fixed composition. His hypothesis states that equal volumes of all gases, at the same temperature and pressure, contain the same number of elementary molecules.

Modern Atomic Concepts

Current Particle Concept

Particles are fundamental entities that may or may not be divisible into others, depending on their nature (e.g., electrons are considered elementary particles, while protons and neutrons are composed of quarks).

Current Atom Concept

An atom is an assembly of a central nucleus (composed of protons and neutrons) and electrons moving in orbitals around it.

Current Groups of Atoms Concept

Atoms can form discrete groups called molecules (e.g., H₂O) or large, extended structures like clusters or networks (e.g., diamond, ionic crystals).

Current Element Concept

An element is a substance formed by a single type of atom, characterized by its unique number of protons (atomic number).

Current Compound Concept

A compound is a substance formed by more than one type of atom chemically bonded together in a fixed ratio.

Chemical Composition and Formulas

Centesimal Composition

The centesimal composition (or percentage composition) of a pure chemical substance expresses the mass percentage of each of its constituent elements.

Empirical Formula

The empirical formula indicates the simplest whole-number ratio of atoms of each element present in a compound.

Molecular Formula

The molecular formula indicates the exact number of atoms of each element present in a molecule of a compound.